In contrast to X-ray transmission imaging, conventional Magnetic Resonance Imaging (MR or MRI) is intrinsically less suited for the depiction and segmentation of cortical bone structures. Besides musculoskeletal applications, MR bone imaging has gained increased importance for applications like MR-based attenuation correction in PET (Positron Emission Tomography)/MR, MR-based radiation therapy planning, and MR-guided focused ultrasound.
Because of low proton density (−20% of water) and short signal lifetimes (T2 ˜0.4 msec at 3 T), there are challenges to apply MRI techniques for depiction of solid bone structures. Conventional gradient echo, or spin echo pulse sequences with echo times (TE) in the millisecond-range are too slow for meaningful bone signal detection. Ultra-short echo time (UTE) pulse sequences with center-out k-space sampling starting immediately following the RF excitation enable fast enough MR data acquisition to capture the rapidly decaying bone signals. In order to selectively depict and segment bone, long T2 suppression methods (like echo subtraction and/or saturation pre-pulses) can be applied. However, these methods are typically slow and suffer from robustness and accuracy issues. Alternatively also atlas-based methods have been presented which add anatomical prior knowledge. However, these methods are less flexible in handling patient abnormalities, e.g. due to pathologies.